Both stem cells and progenitor cells hold significant promise as cell-based therapies. These cells have the unique ability to undergo self-renewal and to differentiate into a variety of different cell types, allowing the continued delivery of therapy for a prolonged period. Added benefits include, the use of the patient's own cells, highly controlled in vitro manipulation of these cells, and the ease of delivery. Little is known, however, regarding the behavior of these progenitor cells in vivo, their specific recruitment to tumors and the magnitude of in vivo cell trafficking. Utilizing a number of in vitro and in vivo imaging approaches, this proposal seeks to 1) determine mechanistic aspects that mediate previously identified apparently selective recruitment of progenitor cells to tumors, and 2) quantitate recruitments for different cells types, tumors and tumor microenvironments with regard to the latter we have recently observed highly efficient homing and retention capabilities. We hypothesize that there exist unique tumor endothelium/progenitor cell interactions that mediate attachment, transmigration and differentiation of the former in tumors and in particular the tumor microvasculature through vasculogenesis. In preliminary feasibility studies we have developed a number of tools that will allow us to interrogate tumor endothelium/progenitor cell interactions in vitro and in vivo. These tools include 1) isolation and characterization of differentiated endothelial cells (murine heart, murine lung and murine Lewis Lung carcinoma derived endothelium) and a panel of progenitor cells (murine C17.2 neuronal progenitors, murine CD34+ hematopoietic stem cells and CD34+/Flk-1+ endothelial progenitors), 2) in vitro flow chambers, 3) methods for imaging progenitor cells in vivo by optical, nuclear and MR imaging methods and 4) methods for isolation of tumor homed cells for comparative genomic analysis. Together, these techniques, and the expertise of a number of collaborators, will provide a powerful approach in understanding the mechanisms that target progenitor cells to tumors. Understanding the mechanisms that mediate progenitor cell recruitment both at a cellular and a molecular level, along with the ability to manipulate these events directly and promote increased recruitment of progenitor cells to the target tumor will provide a springboard to rapidly improved and more specific cell-based therapies for the direct treatment of tumor.